Replaces the explicit 'Load more' button on both library list pages with
an IntersectionObserver-based sentinel that triggers fetchNextPage
automatically as the operator scrolls near the bottom (300px rootMargin).
New InfiniteScrollSentinel component:
- Stays disabled while a fetch is in flight (prevents repeat firing on
tall pages or fast scrolls).
- Tears down its observer when hasNextPage flips false.
- Defensive against environments without IntersectionObserver (jsdom);
tests provide a synchronous mock.
Also fixes a pre-existing test setup gap on the albums page test:
AlbumCard renders LikeButton which calls useQueryClient(), and the
page.test.ts wasn't wrapped in a QueryClientProvider — added the same
useQueryClient mock the AlbumCard.test.ts already uses.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
